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Comparing strain gauges to piezoe-
lectric sensors
Introduction
These two transducer technologies provide great industrial metrological so-
lutions.
They are widely used across different industries for almost the same appli-
cations; however, there are differences between them which includes: per-
formance factors such as resolution, accuracy, and susceptibility to envi-
ronmental errors; utilization factors such as ease of measurement, signal
transmission, equipment interfaces, and physical characteristics.
This article discusses the various points that compare and contrast these
two types of sensors; the information will be very useful to system design-
ers in selecting components for various applications and also for educa-
tional purposes of instrumentation and control systems.
Principle of Operation
A strain gauge sensor works by transforming the quantity to be sensed into
an elastic deformation of the strain gauge. These quantities could be static
or dynamic load, weight, force, acceleration, and pressure. This defor-
mation under the influence of the load causes a change in the dimensions
of the strain gauge material; the deformation then causes a resultant
change in the electrical resistance of the strain gauge material.

The changes in electrical resistance are thereby indicative of the magnitude
of the applied force. The mathematical formula that relates the changes in
dimension to change in length is shown below.
The changes in resistance are then measured by transforming it to an
equivalent change in voltage. A Wheatstone bridge setup is the most com-
monly used technique for a change resistance to a change in voltage trans-
formation.
The strain gauge(s) is/are included into one or more arms of the Wheat-
stone bridge so that changes in its resistance under physical loading
causes a change in the voltage at the output terminals.
The mathematical formula for a quarter-bridge configuration – shown in fig-
ure 1 below – is expressed below. Here dR is the change in resistance of
the strain gauge.

Figure 1. A Quarter-Bridge
Configuration
This output voltage is very small (mV), hence the output analog voltage sig-
nal is always passed through a signal conditioning circuit that amplifies and
filters it. Therefore, basically, the strain gauge sensor detects the physical
force and produces an electrical output that is proportional to the load.
A piezoelectric sensor makes use of a piezoelectric material that trans-
forms the quantity to be sensed into an electric charge. The quantity of the
electric charge is thereby indicative of the magnitude of the applied load.
The piezoelectric material acts like a mechatronic component: it deforms
elastically and mechanically under the influence of the applied load in order
to cause a disorientation of the dipoles within its crystalline structure; there-
fore, an electrode that is appropriately connected across the surfaces of
this piezoelectric material will sense the movement of electric charges. This
shown in figure 2 below.

Figure 2. Piezoe-
lectricity Generation
The mathematical formula for the amount of charge produced is expressed
in the formula below.
The electrical charge between the electrodes is then supplied to a charge
amplifier. The amplifier, depending on the system design, could either see
the piezoelectric sensor as a charge source or a voltage source.
The former is most preferred and it involves the use of an operational am-
plifier; this helps to overcome the effects of stray cable, sensor, and ampli-
fier capacitances. Also, with modern microelectronic techniques, the charge
amplifier can be embedded inside the transducer to shorten the gap be-
tween the electrode and the amplifier.
Furthermore, there are charge leakages caused by the internal resistance
component of the material. This resistance is very large, hence developed
charges decay after a period of time; the charge amplifier caters for this by
shortening the decay period. The voltage output of the amplifier can then
be processed by analog or digital techniques to indicate the sensed quan-
tity.
Also, a piezoelectric sensor can also act as a piezoelectric actuator in a
process called the reverse piezoelectric effect. This effect is observed

when there is a supply of alternating voltage to the sensor, the piezoelectric
material starts to vibrate.
Read:
 Go To Guide for Force and Weight Measurement Resources
 Measuring Forces in the Force Shunt
 Connecting a Force Sensor to a DAQ
Material of Construction
Strain gauge sensors use the strain gauge element as the underlying
mechanism; the types used include thin-film, foil, and semiconductor strain
gauges. They are made from materials such as copper-nickel, nickel chro-
mium, platinum-tungsten alloys, and silicon.
Piezoelectric sensors use a piezoelectric material as its underlying mecha-
nism. These materials can be naturally occurring crystals like Rochelle salt
and quartz. They could also be synthetically made materials which are of
two types: crystalline types such as Lithium Sulphate and Ammonium Di-
Hydrogen phosphate, and polarized ferroelectric ceramics such as Barium
Titanate.
Method of Fabrication
Bonded strain gauges are fabricated by attaching the length of the conduc-
tive strip to a thin backing. This process is called micromachining, in which
the additive material – such as silicon, for a semiconductor strain gauge –
is placed on a substrate – a non-conductive thin backing made from poly-
mers. This process usually involves deposition of conductive materials on
thin films, patterning which arranges the conductor in a grid pattern, and
etching to remove the patterned parts.
The strain gauge is then carefully bonded to the surface of the sensors
structural element by agents like adhesives; to properly perform bonding,
the surface needs to be prepared through the steps of cleaning, smoothing,
roughening, and marking.
The bonded strain gauge is then protected against external mechanical and
chemical damages by hermetic sealing. It should be noted that there are

also non-bonded strain gauges but the common and widely used is the
bonded type that is easily embedded and compact.
Natural piezoelectric materials like quartz simply need to be cut by very fine
precision tools to the required dimension that fits the application. However,
these natural single crystal materials have very bad crystal stability and a
limited degree of freedom.
The synthetic piezoelectric materials like ferroelectric ceramics have ran-
domly oriented internal electrical dipoles within their crystal structure.
Therefore there is a need for them to be polarized; the process to this is
called poling. Poling is done by firstly heating the potentially piezoelectric
powder material to a temperature level higher than the Curie point.
The ferromagnetic properties of the crystal break down at this Curie point
temperature. The next step to polling is then to apply a strong DC electric
field of several kV/mm to the heated material and allow it to cool under this
field. The result is the polarization of the material: the redistribution of the
dipoles in the direction of the applied field. Hence a strong piezoelectric
property is formed.
After cooling the electric field is removed and the material maintains the di-
pole orientation. Figure 3 below explains this better.
Figure 3. The Poling Process
The most important thing that is considered during poling is the geometry of
the crystal, especially when applying the electric field; this greatly affects
the sensitivity of the material.

The polarized material is later supported by a substrate which could also
serve as the outer electrode of the piezoelectric sensor. The material is
fixed such that the direction of application of the deforming stress is per-
pendicular to the direction in which charges are generated – that is the lo-
cation at which the electrodes are placed. This is shown in figure 4 below.
Figure 4. Direction of Stress and
Direction of Movement of Charges
Geometric Shapes and Designs
Strain gauge sensors can be categorized based on the geometric shape of
the structural housing unit to which the strain gauge(s) is/are fixed. This
can be in the forms of a beam, S-shape, disc canisters, and planar beams;
the piezoelectric sensors can also be designed to be housed in these types
of shapes, it all depends on the application requirements.
Both types of sensors can be used for multi-axial applications, they can
support compression, shear, or bending stresses. Piezoelectric sensor de-
signs in comparison to strain gauges’ are compact, small, and have a rug-
ged construction.
Sensor Characteristics

These characteristics are provided in the transducer electrical datasheet
specifications by the original equipment manufacturers. Some of them in-
clude:
 The Force Range: This includes the rated capacity. The force range
of the piezoelectric sensor is about 5KN to 1MN while the strain
gauge sensors have a range of 5N to 40MN.
 Loading Conditions: Piezoelectric load cells can only support dy-
namic loads such as vibrations, accelerations, and dynamic pressure
measurements; strain gauge load cells can support both static and
dynamic loads.
 Creep: strain gauge sensors have very low and insignificant drift in
output when subjected to a load for a long time; piezoelectric sensors
have very large drift in output value which results in errors for long
time measurements.
 Stiffness: piezoelectric sensors have very high stiffness value.
 Resonant Frequency: Unlike strain gauge transducers, piezoelectric
sensors have a higher resonant frequency due to their stiffness. This
value can be as high as 100,000Hz.
 Sealing: both sensors types are designed to have very excellent
seals which offer a high degree of protection and operational safety
in harsh environments. The most common sealing technique is the
hermetic seal.
 Temperature Effects: Both sensors are very sensitive to tempera-
ture changes as it affects the zero-balance, sensitivity, and linearity.
However, various compensation techniques are utilized in the design
of each: strain gauge sensors use self-temperature compensating
gauges; the piezoelectric sensors are able to cater for temperature
effects by the integration with a charge amplifier.
 Repeatability: both sensors can be designed to achieve excellent re-
peatability.
 Linearity: strain gauge sensors have a lower linearity error in com-
parison to piezoelectric sensors.
 Sensitivity: The sensitivity of the piezoelectric sensor depends on
the material used and its geometry; it is rated in Pico Coulombs per
Newton (pC/N). Strain gauge transducer’s sensitivity depends on the
excitation voltage and the rated capacity value; it is rated in millivolts
per volts (mV/V).
Conclusion

The two force transducers are of undeniable importance and any can be
used depending on the requirement of the process they are to be applied
to.
Furthermore, calibration of these devices is also necessary as it yields a
calibration line and can greatly improve the accuracy of measurements by
removing systematic errors.
In summary, piezoelectric sensors offer excellent dynamic measurements
while strain gauge transducers – the most commonly used of the two – of-
fers excellent dynamic and static load measurements.
Sources
 Piezoelectric Sensor, A Method for manufacturing a piezoelectric
sensor and a medical implantable lead comprising such a sensor, US
8,626,313 B2
 Comparative look at strain gauge and piezoelectric sensors by JAC
Chapman, Elexsys
 The Instrumentation Reference Book, Edited by Walt Boyes.
 Jayant Sirohi, Inderjit Chopra , “Fundamental Understanding of Pie-
zoelectric Strain Sensors,” Journal of Intelligent Material Systems
and Structures.
 Force Measurement Glossary by Tacuna Systems.
 Load Cell Material by Tacuna Systems.
Phone Number: 1-800-550-0280
Contact Email: contact@tacunasystems.com
Website: https://tacunasystems.com/